(Probably it is way too late for this, but if IBM had been willing to publicly release PL/S tools back in the 1970s, or at least not use their lawyers to stop others from independently doing that, PL/S and its descendants could have become much more significant systems programming languages.)
UNIX and C only became widespread, because contrary to IBM, DEC, Wang, and many others, AT&T was forbidden to sell UNIX and they started offering it alongside source code for a symbolic price of $100, which was like being free given the prices of other systems.
In any case, that doesn't make C technically better than the alternatives outside AT&T walls, it was just cheaper to get hold of.
But IBM wasn't willing to sell its customers a PL/S compiler.
One reason was that (up until the OCO announcement in 1983) IBM gave its customers the PL/S source code, but not giving them a PL/S compiler discouraged customers from changing the source code, since they couldn't recompile it. Customer modifications had to be made in assembler, which was a lot harder to maintain.
Another reason was to make life harder for competing vendors selling IBM-compatible mainframes, such as Amdahl, Fujitsu and Hitachi.
The PL/8 papers related to IBM's RISC research are quite interesting, before they decided to go with UNIX on RISC instead, as the best way to recover their money, thus switching to C.
Also not all mainframes were written in PL/X variants, there is also NEWP, which already had the notion of unsafe blocks in the late 60's.